Hitherto, it has been known in the rubber industry that when a rubber composition containing a filler, such as carbon black, is produced, a rubber wet master batch is used to improve the workability of the composition, and the dispersibility of the filler in the composition. This rubber wet master batch is a substance obtained by mixing the filler and a dispersing solvent beforehand with each other at a predetermined ratio, dispersing the filler into the dispersing solvent by mechanical force, mixing the resultant filler-containing slurry solution with a rubber latex solution in the phase of the liquids, adding, after the mixing, a solidifier such as an acid thereto to solidify the mixture, and then collecting and drying the mixture. The use of such a rubber wet master batch gives a rubber composition that contains a filler better in dispersibility and is better in rubber physical properties, such as workability and reinforceability, than the use of a rubber dry master batch obtained by mixing a filler with a rubber in a solid phase. The use of such a rubber composition as a raw material makes it possible to produce a pneumatic tire or some other rubber product that is decreased in, for example, rolling resistance, and is excellent in fatigue resistance.
In techniques for producing a rubber wet master batch, a report is made about a technique of using two or more carbon black species together in order to improve the resultant vulcanized rubber in various physical properties.
Patent Document 1 listed below describes a technique of a filler-containing rubber composition in which: a diene rubber is contained in an amount of 10 parts by mass, and a preliminary mixed filler obtained by mixing 80% or more by mass of a hard-class carbon black species preliminarily with 20% or less by mass of a different filler is contained in an amount of 1 to 150 parts by mass; and when the different filler contains a carbon black species, the average particle diameter of primary aggregates of this carbon black species is set to 7/10 or less of that of primary aggregates of the hard-class carbon black species.